The present invention relates to a charging unit used for a contactless transfer of electric power to a mobile unit.
A contactless transfer of power according to the principle of inductive coupling, often referred to as a power transformer, is used for charging accumulators or rechargeable batteries in a mobile electric unit, such as a mobile radio communication device or a cordless telephone. It is also used in the field of automotive engineering. For such contactless power transfer, power is inductively transferred by means of an alternating magnetic field from a charger to the mobile unit, as known from the German Patent Application DE 197 41 279 A1. A primary winding and a secondary winding are arranged on lateral legs of two U-shaped ferrite cores. Other known shapes are rod-shaped or E-shaped ferrite cores. In the case of conventional charging systems used for contactless power transfer, ferrite cores of a primary side and of a secondary side have identical dimensions.
The known embodiments of the ferrite cores have excessively high intrinsic magnetic coupling that is caused by stray inductances between the pairs of poles of each ferrite core. This causes coupling between the primary side and the secondary side to be insufficient. For example, if there is only a coupling of 50% or, in other words, only half of the power is transferred, very high magnetic field strengths occur in the primary core when the cores have identical dimensions. This leads to high core power losses and to a saturation of the core. Identically sized pole faces are also known to have a reduced magnetic coupling when the two pole faces are displaced relative to one another.
In one embodiment, a charging unit used for a contactless transfer of electric power to a mobile unit includes a substantially U-shaped ferrite core. The core has a base area and two leg areas. Each leg area includes a pole face. A primary winding is arranged on the base area of the ferrite core.
In another embodiment, a power absorbing device in a mobile unit used for contactless absorption of electric power from a primary winding located in a charging unit includes a substantially U-shaped ferrite core. The core has a base area and two leg areas. Each leg area includes a pole face. A secondary winding is arranged on the base area of the ferrite core.
In a further embodiment, a charging system used for contactless transfer of electric power from a charging unit to a mobile unit includes a first substantially U-shaped ferrite core which is operable to be located within the charging unit. The first core includes a base area and two leg areas. Each leg area of the first core includes a pole face. A first winding is arranged on the base area of the first ferrite core. The system also includes a second substantially U-shaped ferrite core which is operable to be located within a mobile unit. The second core includes a base area and two leg areas. Each leg area of the second core includes a pole face. A second winding is arranged on the base area of the second ferrite core. The first ferrite core has larger cross-sectional dimensions than the second ferrite core.
In yet a further embodiment, a method of transferring electric power from a primary winding to a secondary winding includes arranging the primary winding on a base area of a first U-shaped ferrite core. The base area of the first core is longer than each leg area of the first core. The method also includes arranging the secondary winding on a base area of a second U-shaped ferrite core. The base area of the second core is longer than each leg area of the second core.
In one embodiment, by arranging a secondary and/or primary winding on a base area of a U-shaped ferrite core, stray inductances can be kept as low as possible. An improved coupling leads to a higher overall efficiency. This permits either an increase in the power that is transferred relative to a core volume or the use of smaller transformers for transferring the same power. Such a transformer saves costs and is advantageous as far as the demands for a far-reaching miniaturization of the components are concerned.
According to a further embodiment, a base area of a U-shaped ferrite core of a secondary side and/or of a primary side is longer than leg areas of the ferrite core, so that the ferrite core has a substantially elongate structural design. This structural design of the core leads to an improved magnetic coupling. In one embodiment, the length ratio of the base area to the leg area is approximately between about 2:1 and about 4:1.
In another embodiment, a charging system used for a contactless transfer of electric power to a mobile unit has a primary-side ferrite core and a secondary-side ferrite core. The primary-side core has larger dimensions than the secondary-side ferrite core. This arrangement allows the core power losses to be kept low and a saturation of the core is avoided, even if an economy-priced standard ferrite material is used.
In a further embodiment, pole faces of a primary-side ferrite core have larger dimensions than pole faces of a secondary-side ferrite core. This arrangement avoids a reduction of the magnetic coupling in the case of a geometric displacement between the two pole faces.